External users not receiving e-mail invites

Got Filr up and running, but one piece evades me. I have tried testing inviting external users (actually me) by sharing a folder with my external address. The invitation never shows up. Additional details:

Filr running on ESXi server in DMZ
Using GroupWise as mail server
Internal users are receiving e- mail notifications from Filr
I can send mail to my outside address via GW directly
External address is not Yahoo or Gmail

Any ideas on how to go about troubleshooting this?